1. Field of the Invention
This invention relates to unidirectional-to-bidirectional switching circuits and, more particularly, to operating telephone ringers from a unidirectional ringing voltage source
2. Description of the Prior Art
In the copending application of T. N. Rao- R. Toumani Ser. No. 638,151, filed of even date herewith, there is disclosed a ringing signal generator comprising a direct current level converter. The high ringing voltage level thus generated on an output capacitor must be interrupted at a 20 Hz rate and utilized to energize a telephone ringer. To this end, a transistor switch is connected in parallel with the ringer load to provide a return path for the alternating current signal in the ringer during alternate half cycles. A second transistor switch is connected in series with the high voltage ringing supply to block current flow from the capacitor to the load and shunt switch. An appropriate connection of the two switches with a diode allows the direct operation of the shunt switch to cause the indirect operation of the series switch. Biasing for the series switch is a direct load on the high voltage supply and dissipates considerable power. The shunt switch can be biased from a lower voltage supply and dissipates less power even though each switch carries a similar level of current. A control signal is supplied to the shunt switch to activate both switches appropriately to supply a ringing voltage output. Both transistor switches must withstand the peak ringing voltage when switched OFF and therefore a high voltage transistor is required for each.